Musk posted videos on Twitter of the aluminum sub being tested at a swimming pool Sunday midafternoon California time. If the tests are successful, the sub would be placed on a 17-hour flight to Thailand.

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Four of the boys were rescued on Sunday, and authorities are now working to replenish air tanks along the cave’s treacherous exit route. They say rescuing the eight remaining boys and their soccer coach could take up to four days.

A spokesman for Musk’s Boring Co. tunneling unit, which has four engineers at the cave, has said Thai officials requested the device, which could potentially help the children through narrow, flooded cave passageways.